AUSTRIAN JOURNAL of EARTH SCIENCES Volume 98 2005

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AUSTRIAN JOURNAL of EARTH SCIENCES [MITTEILUNGEN der ÖSTERREICHISCHEN GEOLOGISCHEN GESELLSCHAFT] an INTERNATIONAL JOURNAL of the AUSTRIAN GEOLOGICAL SOCIETY volume 98 2005

Gerd RANTITSCH & Barbara RUSSEGGER: Organic maturation within the Central Northern Calcareous Alps (Eastern Alps)______

www.univie.ac.at/ajes EDITING: Grasemann Bernhard, Wagreich Michael PUBLISCHER: Österreichische Geologische Gesellschaft Rasumofskygasse 23, A-1031 Wien TYPESETTER: Irnberger Norbert, www.irnberger.net Copy-Shop Urban, Bahnstraße 26a, 2130 Mistelbach PRINTER: Holzhausen Druck & Medien GmbH Holzhausenplatz 1, 1140 Wien ISSN 0251-7493 Austrian Journal of Earth Sciences Volume 98 Vienna 2005 Organic maturation within the Central Northern Calca- reous Alps (Eastern Alps)

Gerd RANTITSCH1) & Barbara RUSSEGGER2) KEYWORDS very low-grade metamorphism Northern Calcareous Alps 1) Department of Applied Geosciences and Geophysics, Montanuniversität Leoben, vitrinite reflectance Austria, A-8700 Leoben, Austria, Email: [email protected] thermal modeling Jurassic orogeny 2) Library of the University of Graz, A-8010 Graz, Austria Eastern Alps

Abstract Organic maturation within the central Northern Calcareous Alps (Eastern Alps) has been investigated using vitrinite reflectance data from Upper Permian to Upper Cretaceous strata of the Stauffen-Höllengebirge Nappe, Dachstein Nappe, Tennengebirge Block and Lammer Unit. Within the Lammer Unit a metamorphic break is observed between the Strubberg Formation and the tectonically overlying Schwarzenberg Complex and Losegg-Hofpürgl slice. Vitrinite reflectance within the Dachstein Nappe and the Stauffen-Höllengebirge Nappe resembles the values within the Schwarzenberg Complex. The thermal overprint of the Tennengebirge Block and the Strubberg Formation is explained by metamorphism due to Late Jurassic imbrication of fault-bounded blocks. A numerical heat flow model explains the thermal overprint of the Dachstein Nappe by sedimentary burial during Mesozoic to Cenozoic times.______

Im Zentralteil der Nördlichen Kalkalpen wurde der Grad der organischen Metamorphose durch die Messung der Vitrinitreflexion in oberpermischen bis oberkretazischen Sedimenten der Stauffen-Höllengebirge Decke, der Dachstein Decke, des Tennengebirge Blocks und der Lammer Einheit erfasst. Der tektonische Kontakt zwischen der Lammer Einheit und den tektonisch aufliegenden Komplexen des Schwarzenbergs und der Losegg-Hofpürgl Schuppe ist durch einen Metamorphosesprung gekennzeichnet. Die Vitrinitreflexion innerhalb der Dachstein- und Stauffen-Höllengebirge Decke entspricht den Werten innerhalb des Schwarzenberg Komplexes. Die thermische Überprägung des Tennengebirge Blocks und der Strubberg Formation wird durch die metamorphe Überprägung während der spätjuras- sischen Imbrikation von störungsbegrenzten Blöcken erklärt. Ein numerisches Wärmeflussmodell erklärt die thermische Überprägung der Dachstein Decke durch die mesozoische bis känozoische sedimentäre Auflast der Schichtfolge.______

1. Introduction The Northern Calcareous Alps (NCA) are an elongated fold- closer insight to thermal processes in higher crustal levels. and-thrust-belt (Linzer et al. 1995) within the Eastern Alps (Fig. However, the central segment of the NCA, the key area of the still 1). The classical tectonic subdivision describes a lower Bavaric, ongoing debate about the internal structure of the NCA an intermediate Tirolic and an upper Juvavic nappe group (Tollmann, 1976 vs. Neubauer, 1994 and Schweigl and (Tollmann, 1976; Mandl, 2000). Whereas the Bavaric and the Neubauer, 1997 vs. Frisch and Gawlick, 2003), is only sparsely Tirolic units involve sediments of a Triassic to Jurassic carbonate covered by organic maturity data. In this contribution, we present platform facing the western Tethyan Ocean, the Juvavic units therefore vitrinite reflectance data to complete the pattern of derived from the platform rim and outer shelf area (Hallstatt organic metamorphism and to give an insight in the thermal Zone). Based on stratigraphic, structural, metamorphic and structure of this section. geochronological data from the central NCA, Frisch and Gawlick (2003) proposed an alternative nappe concept in which the Ju- 2. Geological setting vavic Dachstein Nappe is a subunit of the Tirolic nappe group The Late Triassic facial distribution of the NCA describes a and deeper shelf sediments of the former Hallstatt Zone are carbonate platform (Dachstein platform) faced in the south by the parts of a flysch succession (Hallstatt mélange).______outer shelf and shelf edge realm of the Hallstatt Zone (Haas et al., Because the thermal structure of fold-and–thrust-belts is us- 1995, Gawlick et al., 1999; Mandl, 2000). This facies belt was ually attributed to thrust-related processes like burial by thrust destroyed during the Late Jurassic closure of the adjacent sheets or shear heating along thrust zones (e.g. Warr et al., Meliata Ocean (Neubauer, 1994; Neubauer et al., 2000; Mandl, 1996), detailed metamorphic studies can contribute significantly 2000; Frisch and Gawlick, 2003). Late Jurassic to Eocene to controversies about opposing nappe concepts. It was de- thrusting (Mandl, 2000; Frisch and Gawlick, 2003) and Early to monstrated in numerous studies that the use of organic meta- Middle Miocene block fragmentation (Linzer et al., 1995; Frisch morphic parameters resulted in the construction of the most et al., 1998; Frisch and Gawlick, 2003) determined the present detailed and meaningful metamorphic maps. Moreover, the tectonic structure of the NCA. sensitive and irreversible response of organic matter to subtle In the traditional model (Tollmann, 1976; Mandl, 2000), Late temperature variations and the well-known kinetics of organic Jurassic to Cretaceous north-directed thrusting displaced the maturation allow the calibration of thermal models which give a deeper shelf and platform sediments of the Hallstatt Zone as a 68 Gerd RANTITSCH & Barbara RUSSEGGER

Juvavic Nappe Complex (involving the Dachstein Nappe, Fig. 2) bituminous Carnian (Lunz) coals, implying maximum burial upon the Tirolic Nappe Complex (involving the Stauffen-Höllen- temperatures of c. 170° C (Sachsenhofer, 1987). gebirge Nappe, Fig. 2). Jurassic basins of the Tirolic Nappe According to Frisch and Gawlick (2003) the Late Jurassic Complex containing gravity transported slices (olistoliths) of the imbrication of the accretionary wedge metamorphosed the Hallstatt Zone were overthrusted by the Juvavic Nappe Complex. olistoliths of the Hallstatt Zone. Conodont Alteration Indices The contrary model by Frisch and Gawlick (2003) describes a (ranging from CAI 1.0 to CAI 7.0) within carbonate olistoliths of Callovian to Oxfordian accretionary wedge which was com- the Hallstatt Zone (Gawlick and Königshof, 1993; Gawlick et al., pletely destroyed during the Late Jurassic. The debris of this 1994; Gawlick and Höpfer, 1996) support this model. According wedge was transported towards the north into a radiolaritic flysch to data from the entire NCA, Spötl and Hasenhüttl (1998) basin (Lammer Basin) which subsided on top of the Dachstein concluded that the imbrication is responsible for the variation of carbonate platform. Progressive north-directed thrusting vitrinite reflectance and illite crystallinity in the Upper Permian separated the Dachstein carbonate platform into a lower- and an Haselgebirge. From calcite-dolomite thermometry Gawlick and upper-Tirolic nappe. The Stauffen-Höllengebirge Nappe is Höpfer (1996) supposed maximum burial temperatures of 350° C interpreted as a segment of the lower-Tirolic Nappe whereas the to 490° C at minimum pressures of 6-8 kbar during this event. A Dachstein Nappe is interpreted as a segment of the upper-Tirolic large segment of the wedge (Tennengebirge block of the Ultra- Nappe. During Oxfordian to Tithonian times radiolaritic basins Tirolic nappe sensu Frisch and Gawlick, 2003) was deeply buried developed at the frontal ramp of the upper Tirolic nappe and metamorphosed (Frisch and Gawlick, 2003). Kralik and (Tauglboden Basin) and out-of-sequence upon the Lammer Schramm (1994) explained the lower thermal overprint in the Basin (Sillenkopf Basin). During the Eocene period, out-of- western segment of the Stauffen-Höllengebirge Nappe by a sequence thrusting transported the Dachstein Nappe upon younger metamorphic event which was induced by thrusting sediments of the Hallstatt Zone (Frisch and Gawlick, 2003). combined with fluid circulation. Geochronological data constrain In the western and central part of the NCA there is a general the age of metamorphism between 130 and 140 Ma (Kralik et al., north to south and east to west increase in the metamorphic 1987; Kralik and Schramm, 1994). overprint (Kralik et al., 1987; Krumm et al., 1988; Petschick, 1989; Progressive thrusting was followed by a phase of uplift and Kürmann, 1993; Ferreiro Mählmann, 1994). In this segment, erosion during the Early Cretaceous before the Gosau basins Petschick (1989) and Ferreiro Mählmann and Petschick (1995) (Figs. 1, 2) subsided during Turonian to Eocene times on top of proposed a (pre-tectonic) Permian to Early Cretaceous dias- the nappe stack (Wagreich and Faupl, 1994; Wagreich, 1995, tathermal metamorphism with burial temperatures up to 300° C, 2001; Wagreich and Decker, 2001). a syn-tectonic thermal event during Early Cretaceous to Turonian The Lammer Unit (Tollmann, 1976; Fig. 2) is the key area for the times with burial temperatures of c. 250° C, and a posttectonic evaluation of the Late Jurassic tectonometamorphic history. At thermal event acting in the Early Cenozoic period. In the the southern margin of this unit, 250 m thick Upper Jurassic northeastern part of the NCA an eastward increase in the ther- radiolaritic flysch sediments (Strubberg Formation) were mal overprint is indicated by sub-bituminous to high-volatile deposited within the Lammer Basin (Gawlick et al., 1999), which

Figure 1: Location of the study area within the Alps. 69 Organic maturation within the Central Northern Calca-reous Alps (Eastern Alps) itself is a part of the upper-Tirolic unit of Frisch and Gawlick values from Gosau sediments (Sachsenhofer, unpublished). (2003). These sediments are tectonically (Gawlick et al., 1990; Petromod 9.0 1D software of IES, Jülich was used to model Schweigl and Neubauer, 1997) separated towards the north from paleo-heat flow during burial heating of the NCA. Input data more than 1000 m thick Scythian to Liassic strata of the include the thickness of stratigraphic units, the physical Schwarzenberg Complex (Tollmann, 1976; Gawlick et al., 1990; properties of their lithologies, the temperature at the sedi- Gawlick, 1996; Frisch and Gawlick, 2003). According to Gawlick ment–water interface, information on the stratigraphy and litho- et al. (1990) the Schwarzenberg Complex is an olistolith which logy of the modelled section, and the temporal evolution of slided gravitationally into Upper Jurassic flysch sediments of the the heat flow at the base of the sedimentary succession. This Lammer Basin. Other authors postulated a classical Juvavic information was used to reconstruct decompacted subsidence position of the Schwarzenberg Unit (Tollmann, 1976; Plöchinger, histories and the temperature field through time, calibrated by 1980; Schweigl and Neubauer, 1997; Mandl, 2000). observed vitrinite reflectance values. For the calculation of vitrinite reflectance, the kinetic EASY%Ro method of Sweeney 3. Samples and methods and Burnham (1990) was applied. Temperature and time are Organic maturity was determined in samples from the Scythian considered the prime factors controlling vitrinite maturation. In Werfen Formation, from the Carnian Raibl Formation, from the the used algorithm heat enters at the base of the decompacted Upper Jurassic Strubberg Formation and from Upper Cretaceous stratigraphic succession and is transferred conductively accor- Gosau sediments. Random vitrinite reflectance values were ding to temperature gradients. Only forced (i.e. compaction measured on sections polished perpendicular to the foliation induced) convection is directly taken into account. Effects of free under oil immersion in non-polarized light at a wavelength of 546 convective fluid flow systems were not simulated. Effects of nm. Some samples were characterized by the apparent maxi- thrusting within the succession were also not taken into account. mum and minimum vitrinite reflectance values measured in polarized light. The data base (Tab. 1) is completed by published 4. Vitrinite reflectance data vitrinite reflectance data from the Upper Permian Haselgebirge Vitrinite reflectance values from the Upper Jurassic Strub- (Spötl and Hasenhüttl, 1998), by vitrinite reflectance data from berg Formation range between 2.0 and 3.8 %Ro (Rantitsch et al., Carnian strata of the Grünau I well (Colins et al., 1992), by vitrinite 2003). Significantly lower values (0.8 to 1.1 %Ro) were mea- reflectance data from the Upper Jurassic Strubberg Formation sured in samples from the Carnian Raibl Formation of the (Rantitsch et al., 2003), and by unpublished vitrinite reflectance Dachstein and Stauffen-Höllengebirge Nappe (Tab. 1, Fig. 2).

Figure 2: Vitrinite reflectance (%Ro) within the central Northern Calcareous Alps (basemap redrawn from Mandl, 2000). The data give an average from all samples of one sample locality (Tab. 1). One sample locality within the Tennengebirge Range (circle symbol with a central point) is characterized by a %Rmax value as a mean value of two samples (Tab. 1). 70 Gerd RANTITSCH & Barbara RUSSEGGER

Although the sample grid does not cover the southern segments is characterized by 1.5 %Ro. The samples within the Tennen- of the nappes, mapped CAI (Gawlick et al., 1994) and illite cry- gebirge Range show a high vitrinite reflectance of 4.7 %Rmax stallinity (Kralik et al., 1987; Kralik and Schramm, 1994) values (mean of two samples, see Tab. 1, Fig. 2). Upper Cretaceous suggest similar vitrinite reflectance values in the uncovered Gosau sediments are characterized by a vitrinite reflectance regions. Carnian samples from the Losegg-Hofpürgl slice (Fig. 2) of 0.5 to 0.6 %Rmax (Fig. 2). Samples from two gypsum- and from the Schwarzenberg Complex (Fig. 2) show a vitrinite anhydrite mines (Fig. 2) indicate a mean vitrinite reflectance reflectance of 1.1 and 1.0 %Ro, respectively. One sample from of 1.9 to 2.0 %Ro in the Upper Permian Haselgebirge (Spötl the Scythian Werfen Formation of the Schwarzenberg Complex and Hasenhüttl, 1998).

Table 1: Mean random (Ro), maximum (Rmax) and minimum (Rmin) vitrinite reflectance within the central Calcareous Alps (s= standard deviation, N= number of measurements). Samples S are unpublished values of R. Sachsenhofer. Sample Co is derived from Colins et al. (1992), samples Sp from Spötl and Hasenhüttl (1998), and samples R from Rantitsch et al. (2003). The coordinates (X, Y) refer to the Austrian M31 coordinate system. Organic maturation within the Central Northern Calca-reous Alps (Eastern Alps)

5. Discussion composed of olistoliths floating in radiolaritic flysch sediments, filled the Lammer Basin on top of the Upper Triassic Dachstein 5.1 Heating of the Tennengebirge block carbonate platform, which was subsequently displaced as a and Strubberg formation Tirolic Nappe Complex sensu Frisch and Gawlick (2003). Vitrinite reflectance values from the Strubberg Formation range However, vitrinite reflectance values from the Carnian Raibl between 2.0 and 3.8 %Ro (Tab. 1). Based on the time- Formation of the Dachstein and Stauffen-Höllengebirge Nappe independent temperature regression of Barker (1988) these (subunits of the Tirolic Nappe Complex sensu Gawlick and values correspond to paleotemperatures of 220° C to 290° C. Frisch, 2003) range between 0.8 and 1.3 %Ro and indicate In contrast, Carnian samples from the Losegg-Hofpürgl slice (an olistolith at the southeastern margin of the Lammer Unit, Tollmann, 1976, Fig. 2) and from the Schwarzenberg Complex show a vitrinite reflectance of 1.1 and 1.0 %Ro. These values correspond roughly to CAI values of this region which describe a top- to-bottom increase of CAI 1.0 to CAI 2.5 (Gawlick, 1997). Gawlick (1997) correlated them with maximum burial temperatures below 180° C.___ To evaluate the thermal history it is important to note that the model of Frisch and Gawlick (2003) ex- pects a burial heating pattern (i.e. an increase of the metamorphic rank with increasing burial depths) within the Lammer Basin (Gawlick, 1997). In a consistent model, the Lammer Basin is composed of olistoliths (involving the Schwarzenberg Com- plex) floating in an unmetamor- phosed sedimentary matrix (Strub- berg Formation). However, the Strubberg Formation is tectonically separated from the overlying Schwarzenberg Complex (Gawlick et al, 1990; Gawlick and Frisch, 2003). This fact and the observation of significantly contrasting vitrinite reflectance values in these units argue against the supposition of Frisch and Gawlick (2003). In our concept, the high thermal overprint of the Strubberg Formation must be explained by a deeper burial of a tectonically separated block or by the loading of a pile of sediments on top of the exposed succession of the Strubberg Formation which was removed tectonically after the accumulation of the olistoliths of the Schwarzenberg Complex. If the latter hypothesis is correct, a more Table 2: Input parameters for the 1-D thermal model compiled from Mandl (2000), Gawlick and Frisch than 2000 m thick succession, (2002), Wagreich (1988, 1995, 2001), Wagreich and Decker (2001) and Frisch et al. (2001). 72 Gerd RANTITSCH & Barbara RUSSEGGER therefore a supposition of a higher metamorphic unit (Strubberg Cretaceous temperature maximum (Kralik et al., 1987; Hejl and Formation) above a lower metamorphic unit. This observation Grundmann, 1989; Kralik and Schramm, 1994; Spötl et al., 1998). contradicts the assumption of an autochthonous position of The simplified stratigraphical model includes Upper Permian the Lammer Basin above the Dachstein carbonate platform. evaporites (Haselgebirge), Triassic carbonate platform We suppose therefore, that Late Jurassic thrusting fragmented sediments (Wetterstein and Dachstein limestones), Lower to the Lammer Basin during the time of flysch sedimentation. As Upper Jurassic pelagic (Adnet, Klaus and Strubberg Formation) a result, the tectonically detached stratigraphic base of the and Upper Jurassic carbonate platform successions (Plassen basin (with the sediments of the Strubberg Formation) expe- limestone), transgressively overlain by Turonian to Eocene rienced a deeper burial than the sediments of the Dachstein Gosau sediments and Oligocene-Miocene fluvial gravel of the and Stauffen-Höllengebirge Nappes and the tectonically Augenstein Formation. Major subsidence events occurred during overlying Schwarzenberg Complex.______the Late Jurassic (Strubberg Formation), the Late Cretaceous Late Jurassic imbrication deeply buried the Tennengebirge (Gosau Basins) and the Oligo-/Miocene (Augenstein Formation). Block south of the Lammer Unit (Frisch and Gawlick, 2003), an Thicknesses of units (Tab. 2) are based on published thickness ultra-Tirolic unit according to Frisch and Gawlick (2003). Rather values (Mandl, 2000; Gawlick and Frisch, 2002; Wagreich, 1988, high vitrinite reflectance values of 4.1 to 5.3 %Rmax characterize 1995, 2001; Wagreich and Decker, 2001; Frisch et al., 2001) and the Carnian strata of this block (Tab. 1). These values resemble do not account for tectonic reductions. Rmax values from the Strubberg Formation (Tab. 1). Hence, the Several models were calibrated by modifying heat flow and the Strubberg Formation at the southern margin of the Lammer Unit thickness of eroded sediments until a satisfactory fit between shows the same metamorphic overprint as the Tennengebirge measured and calculated vitrinite reflectance values was ob- Block and therefore, may be interpreted as a part of the Ten- tained. In order to respect the temporal constraints, heat flow has nengebirge Block (Tollmann, 1976; Gawlick et al., 1990; to be raised during Late Jurassic times. Models without intro- Schweigl and Neubauer, 1997).______duction of elevated heat flows during this time result in a Late Consequently, we see in the area of the central NCA evidences Cretaceous temperature maximum which is not mirrored by geo- for a juxtaposition of tectonic blocks which are characterized by chronological data. If the model includes a 2000 m thick Upper different thermal histories. Accepting the model of Frisch and Jurassic succession of olistoliths floating in radiolaritic flysch Gawlick (2003) a metamorphic overprint due to Late Jurassic sediments (sedimentary filling of the Lammer Basin, Gawlick and imbrication of fault-bounded blocks in an accretionary prism can Frisch, 2003) no acceptable fit between observed and calcu- explain the pattern of metamorphism within the Tennengebirge lated vitrinite reflectance values is achieved (Fig. 3). In contrast, Block and Strubberg Formation. Radiometric age data (Kralik et al., 1987; Hejl and Grundmann, 1989) and isotopic evidences (Spötl et al., 1996, 1998) support this model.______

5.2 Heating of the Dachstein and Stauffen- Höllengebirge nappe North of the Lammer Zone, vitrinite reflectance values from the Carnian Raibl Formation of the Dachstein and Stauffen- Höllengebirge Nappe range between 0.8 and 1.3 %Ro. In both nappes the values increase slightly towards the south and suggest a break in vitrinite reflectance across the nappe boundary. Although, the incomplete spatial coverage of the sample grid does not allow a final interpretation, it is interesting to note that this observation resembles the observations of coalification breaks between individual Bavaric nappes east of the study area (Sachsenhofer, 1987).______Vitrinite reflectance within the Upper Cretaceous Gosau Basins constrains a stratigraphic trend of deceasing organic maturity within the Permian to Cretaceous sedimentary succession of the Dachstein and Stauffen-Höllengebirge nappe. This observation indicates a heating of the succession during Mesozoic to Cenozoic sedimentary burial.______Figure 3: Measured (dots) and calculated (line) vitrinite reflectance values of a thermal model characterized by a heat flow of 60 mW/m2 during 5.3 Thermal modeling of the Dachstein nappe the Late Jurassic period. The Strubberg Formation is assumed to be 2000 A one-dimensional numerical model of the Dachstein Nappe m thick (Gawlick and Frisch, 2003). This scenario results in a bad calibration and therefore has to be modified. A Late Jurassic to Early reconstructs the heat flow history of the Dachstein carbonate Cretaceous temperature maximum cannot be modelled by the platform which is characterized by a Late Jurassic to Early assumption of lower heat flow values. 73 Organic maturation within the Central Northern Calca-reous Alps (Eastern Alps)

Figure 4: Burial history, temperature history and heat flow model of the Dachstein Nappe in the central Northern Calcareous Alps (for thickness data and other input parameters see Tab. 2). Dashed curves are iso-temperature lines. In the right part of the figure measured (dots representing mean vitrinite reflectance within the stratigraphic layer) and calculated vitrinite reflectance values (line) calculated on the basis of burial and temperature history using the EASY%Ro method are plotted versus depth. The model explains vitrinite reflectance values measured in the Werfen Formation of the Schwarzenberg Complex and the Upper Permian Haselgebirge from the Hallstatt Zone (open dots representing mean vitrinite reflectance within the stratigraphic layer). a model which describes the Late Jurassic burial history of the 6. Conclusions Dachstein Nappe without incorporation of several hundred meter Vitrinite reflectance data give information about the tecto- thick Upper Jurassic mass-flows (Tab. 2), results in a fit between nothermal evolution of the central segment of the Northern Cal- observed and calculated vitrinite reflectance values (Fig. 4).___ careous Alps. The following conclusions have been reached: Within a narrow variability range, the model is characterized 1. The data do not suggest an autochthonous position of the by an increasing heat flow (up to 90 mW/m2) during Jurassic Lammer Basin on top of the Dachstein carbonate platform._ times. Geologically, reasons for this increase are poorly und- 2. A break in vitrinite reflectance between the Strubberg For- erstood. However, the Jurassic opening of the South Penninic mation and the overlying Schwarzenberg Complex argues (Piemontais-Ligurian) ocean north of the NCA may provide an against a sedimentary contact of these units.______explanation. Although the used algorithm does not allow the 3. Metamorphism within the Tennengebirge Block and the incorporation of a convective heat transfer, isotopic evidence Strubberg Formation is explained by a metamorphic over- (Kralik and Schramm, 1994; Spötl et al., 1996, 1998) demon- print due to the Late Jurassic imbrication of fault-bounded strates the contribution of hot circulating fluids. Therefore, the blocks. apparent increase in heat flow might have been also an effect 4. A numerical heat flow model explains the thermal overprint of heat transfer by migrating fluids which influenced the tem- of the Dachstein carbonate platform by Mesozoic to Cenozoic perature field. During Cretaceous times the heat flow decreased burial heating with an elevated heat flow during Jurassic to moderate values (60 mW/m2) until the low Oligocene to recent times. heat flow (35 mW/m2) of an orogenic front (Sachsen-hofer, 2001) 5. Organic maturation within the Schwarzenberg Complex and was reached.______Losegg-Hofpürgl slice of the Lammer Unit can be explained The findings of the 1D-modeling approach demonstrate that by the burial and heat flow history of the Dachstein Nappe. the Mesozoic to Cenozoic burial heating by the known sedimentary column can explain the thermal overprint of the Dachstein Acknowledgements Nappe. The thermal model fits vitrinite reflectance values which This study was financially supported by the Austrian Science were measured within Scythian and Carnian sediments of the Fund (FWF) due to grant P10277. R. Sachsenhofer provided Schwarzenberg Complex and Losegg-Hofpürgl slice (Fig. 4). A us with unpublished vitrinite reflectance data. We thank M. metamorphic overprint of these blocks due to deep Late Jurassic Wagreich (Wien), R. Ondrak (Potsdam) and an anonymous burial in an accretionary prism is therefore highly unlikely._____ reviewer for constructive criticisms.______74 Gerd RANTITSCH & Barbara RUSSEGGER

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